Abstract

The effect of quantum mechanics (QM) on the details of the nucleation process is explored employing Ne clusters as test cases due to their semi-quantal nature. In particular, we investigate the impact of quantum mechanics on both condensation and dissociation rates in the framework of the microcanonical ensemble. Using both classical trajectories and two semi-quantal approaches (zero point averaged dynamics, ZPAD, and Gaussian-based time dependent Hartree, G-TDH) to model cluster and collision dynamics, we simulate the dissociation and monomer capture for Ne8 as a function of the cluster internal energy, impact parameter and collision speed. The results for the capture probability Ps(b) as a function of the impact parameter suggest that classical trajectories always underestimate capture probabilities with respect to ZPAD, albeit at most by 15%–20% in the cases we studied. They also do so in some important situations when using G-TDH. More interestingly, dissociation rates kdiss are grossly overestimated by classical mechanics, at least by one order of magnitude. We interpret both behaviours as mainly due to the reduced amount of kinetic energy available to a quantum cluster for a chosen total internal energy. We also find that the decrease in monomerdissociation energy due to zero point energyeffects plays a key role in defining dissociation rates. In fact, semi-quantal and classical results for kdiss seem to follow a common “corresponding states” behaviour when the proper definition of internal and dissociation energies are used in a transition state model estimation of the evaporation rate constants.

Received 28 March 2012Accepted 06 June 2012Published online 02 July 2012

Acknowledgments:

M.M. acknowledges the financial support provided by the “Rientro dei Cervelli” scheme funded by the Italian Ministry for the University and Reserach (MIUR), a Visiting Research grant awarded by the Université Paul Sabatier, as well as the hospitality at the Laboratoire Collisions Agrégats Réactivité-CNRS. We also acknowledge the calculation facilities provided by CALMIP. N.H. acknowledges support from the French National Research Agency under Programme Blanc (ANR-08-BAN-0146-02, DYNHELIUM).